Nickel-metal hydride batteries have been used heretofore as rechargeable batteries for hybrid automobiles mainly from the viewpoints of costs and weight saving. Lithium ion secondary batteries have high voltage per battery and also have high energy density, and the application of such batteries is expected to realize further reduction of weight.
Batteries for automobiles running on electricity alone such as electric vehicles are to be made of materials having high energy density in order to ensure a long driving distance per charging. The use of graphite materials for negative electrodes has been widely studied.
On the other hand, hybrid automobile batteries are a system which has a small capacity of batteries mounted on automobiles and therefore needs to regenerate energy by regenerative braking. In this system, batteries having high input and output densities during charging and discharging are demanded, and the use of amorphous carbon particles represented by hard carbons has been studied.